A transmission apparatus includes a plurality of plug-in units. In order to transmit and receive signals among these plurality of plug-in units without signals being lost, electrical components, optical components, and the like are mounted on the respective plug-in units, and synchronization of clock signals and frame pulse signals used in the respective plug-in units is established.
As a method of establishing synchronization of clock signals among the plurality of plug-in units, the following method is known. For example, a reference clock signal as a reference of synchronization between plug-in units is sent from a first plug-in unit among the plurality of plug-in units to a second plug-in unit that is different from the first plug-in unit, and a clock signal generated by the second plug-in unit is returned to the first plug-in unit. The first plug-in unit compares a phase of the returned clock signal with that of the reference clock signal, and feedbacks a result of the comparison to the second plug-in unit. The second plug-in unit corrects the phase of the clock signal based on the fed back result of the comparison.
In addition, a method of using an operation circuit for phase comparison in relation to synchronization of clocks to control a variation amount of the clocks is also known.
Examples of related art documents include Japanese Laid-open Patent Publication Nos. 2008-252824, 10-200515, and 10-257038.